Wednesday, May 4th: “A” DayAgenda (1:05 release)

Collect Take-Home QuizSection 15.2: “Acidity, Basicity, and pH”

Self-ionization constant of H2O, Kw, neutral, pH, indicatorHomework:

Section 15.2 review, pg. 547: #1-3, 5, 7-12Concept Review: “Acidity, Basicity, and pH”

Looking Ahead:Titration Lab: Thursday, May 12th

Chapter 15 Webquest/concept review due:

Wed, May 18th

Self-Ionization of Water Water can act as an acid and as a base. This means that a water molecule can

either give or receive a proton.

Base Acid CA CBA pair of water molecules are in equilibrium with two ions—a hydronium ion and a hydroxide ion—in a reaction known as the self-ionization of water.

Self-Ionization of Water

The equilibrium expression for this reaction is simply Keq = [H3O+][OH−]

This equilibrium constant, called the self-ionization constant, is so important that it has a special symbol, Kw.

Base Acid CA CB

Self-Ionization of Water

In pure water, the two ions must share the same concentration.

Experiments show that this concentration is1.00 × 10−7 M at 25°C.

[H3O+] = [OH−] = 1.00 × 10−7 M

Kw = [H3O+] [OH-]

Kw = 1.00 X 10-14

Base Acid CA CB

Self-Ionization of WaterThe product of these two ion concentrations is

always a constant.

Anything that increases one of the ion concentrations decreases the other.

If you know one of the ion concentrations, you can calculate the other.

Self-Ionization of WaterThe concentration of hydronium ions (H3O+) in

a solution expresses its acidity. The concentration of hydroxide ions (OH-) in a

solution expresses its basicity.

Sample Problem A, pg. 541Determining [OH-] or [H30+] using Kw

What is the [OH-] in a 3.00 X 10 -5 M solution of HCl?HCl is a strong acid and ionizes completely:

HCl (g) + H2O (l) H3O+ (aq) + Cl- (aq)

3.00 X 10-5 M 3.00 X 10-5 MBecause 1:1 ratio, [HCl] = [H3O+] = 3.00 X 10 -5 M

Kw = [OH-] [H3O+] = 1.00 X 10 -14

1.00 X 10 -14 = [OH-] (3.00 X 10 -5 M)

[OH-] = 3.33 X 10-10 M

Additional PracticeWhat is the [H3O+] in a solution of NaOH whose

concentration is 3.75 X 10 -2 M?NaOH is a strong base and ionizes completely:

NaOH (s) Na+ (aq) + OH- (aq) 3.75 X 10 -2 M 3.75 X 10 -2 MBecause 1:1 ratio, [NaOH] = [OH-] = 3.75 X 10 -2 MKw = [OH-] [H3O+] = 1.00 X 10 -14

1.00 X 10 -14 = 3.75 X 10 -2 M [H3O+]

[H3O+] = 2.67 X 10-13 M

pH and AcidityWhen acidity and basicity are exactly balanced

such that the numbers of H3O+ and OH− ions are equal, we say that the solution is neutral.

Pure water is neutral because it contains equal amounts of the two ions.

Measuring pH

The letters p and H stand for “power of hydrogen”.

pH can be calculated by the following mathematical equation:

pH = −log [H3O+]

Measuring pH

Because of the negative sign, as the hydronium ion concentration increases, the pH will decrease.

A solution of pH 0 is very acidic.

A solution of pH 14 is very alkaline.

A solution of pH 7 is neutral.

Measuring pH

The pH equation may be rearranged to calculate the hydronium ion (H3O+) concentration from the pH.

[H3O+] = 10−pH

pH values of some common materials

Sample Problem B, pg. 544Calculating pH for an Acidic or Basic Solution

What is the pH of (a) a 0.00010 M solution of HNO3, a strong acid, and (b) a 0.0136 M solution of KOH, a strong base?

(a): HNO3 is a strong acid and ionizes completely:

HNO3 (g) + H2O (l) → H3O+ (aq) + NO3- (aq)

0.00010 M 0.00010 M

Because 1:1 ratio, [HNO3] = [H3O+] = 0.00010 M

pH = -log [H3O+] pH = -log (0.00010)

pH = 4.00

Sample Problem B continuedWhat is the pH of (a) a 0.00010 M solution of HNO3, a

strong acid, and (b) a 0.0136 M solution of KOH, a strong base?

(b): KOH is a strong base and ionizes completely:KOH (s) → K+ (aq) + OH- (aq)

0.0136 M 0.0136 MBecause 1:1 ratio, [KOH] = [OH-] = 0.0136 MKw = [OH-] [H30+] = 1.00 X 10 -14

1.00 X 10 -14 = (0.0136 M) [H30+] [H30+] = 7.35 X 10 -13 pH = - log [H30+] pH = - log (7.35 X 10 -13)

pH = 12.13

Sample Problem C, pg. 545Calculating [H3O+] and [OH-] from pH

What are the concentrations of the hydronium and hydroxide ions in a sample of rain water that has a pH of 5.05?

[H3O+] = 10−pH

[H3O+] = 10 -5.05

[H3O+] = 8.91 X 10-6 MKw = [H3O+] [OH-] = 1.00 X 10 -14

8.91 X 10 -6 M [OH-] = 1.00 X 10 -14

[OH-] = 1.12 X 10-9 M

Additional PracticeIf the pH of a solution is 8.1, what is the [H3O+] ?

What is the [OH-] in the solution?[H3O+] = 10−pH

[H3O+] = 10 -8.1

[H3O+] = 7.9 X 10-9 MKw = [H3O+] [OH-] = 1.00 X 10 -14

7.9 X 10 -9 M [OH-] = 1.00 X 10 -14

[OH-] = 1.3 X 10-6 M

IndicatorsCertain dyes, known as indicators, turn

different colors in solutions of different pH.

An indicator is a compound that can reversibly change color depending on the pH of the solution or other chemical change.

pH MetersA pH meter is an electronic instrument equipped

with a probe that can be dipped into a solution.

The probe has two electrodes, one of which is sensitive to the hydronium ion.

An electrical voltage develops between the two electrodes, and the circuitry measures this voltage.

The instrument converts the measurement into a pH reading, which is displayed on the meter’s screen.

Indicators vs. pH MetersIndicators

Quick and convenientDo not give very precise results

pH MetersVery preciseMore complicated and expensive

HomeworkSection 15.2 review, pg. 547: #1-3, 5, 7-12Concept Review: “Acidity, Basicity, and pH”

Be ready for a quiz over this section next time…